Lighting systems, e.g. lighting systems for office use where large areas need to be lit up to a rated light flux level, usually comprise a plurality of lighting units (or luminaires, or groups of luminaires), each being configured to light a target area. In combination, the lighting units provide an adequate lighting of the total space.
In view of saving energy, the lighting units may comprise a light sensor (also referred to as a daylight sensor) which takes into account a contribution of daylight to the light flux level in the target area of the lighting unit in addition to the artificial light flux generated by the lighting unit itself. The light sensor, which contains a photoconductive cell, is configured to measure the light flux in the target area of the lighting unit. If the target area of the lighting unit has a sufficient light flux level generated by natural light entering the target area, then the corresponding lighting unit will be controlled through the light sensor to produce no, or a low level of (artificial) light flux in the target area of the lighting unit. Other lighting units in whose target area only a limited natural light flux, or no natural light flux at all enters, will be controlled through their corresponding light sensor to produce a high level of artificial light flux. Accordingly, whether a high natural light flux or a low natural light flux is present in the different target areas, all target areas may be lit at the same light flux level by adding an appropriate artificial light flux.
If the light flux level in a target area of a lighting unit, when using a light sensor, is found to be higher than required, or higher than desired, it should be possible to (manually or automatically) dim the lighting unit. This applies in particular when the natural light flux is relatively low, e.g. in a dark section of a building area, in periods of dark weather, or at nighttime. Dimming of lighting units may be done by adding a manually or automatically operated dimming element to each lighting unit, the dimming element overriding the light sensor. Also, digital control systems may be used providing manual or automatic dimming of one or more lighting units.
Disadvantages of the known dimming arrangements in use are the required number of components, the complexity of the lighting system, the associated costs including installation costs, difficulties in commissioning of the lighting system, etc.